Improved functional recovery after ischemic preconditioning in the globally ischemic rabbit heart is not mediated by adenosine A1 receptor activation.

Experiments were carried out to test the hypothesis that preconditioning reduces the impairment of recovery of cardiac mechanical function and that this effect is mediated by activation of adenosine A1 receptors. Isolated hearts were Langendorff-perfused at 37 degrees C with oxygenated blood and paced at 3 Hz. They were divided into 5 groups, all subjected to 45 min global ischemia followed by one hour of reperfusion: 1) Control hearts (n = 7) which received no treatment or short ischemia before the long ischemia, 2) preconditioned hearts (n = 7), submitted to 5-min zero-flow global ischemia, followed by 5 min reperfusion before the long ischemia, 3) hearts pretreated with sulfophenyltheophylline (SPT 100 microM) before preconditioning and long ischemia (n = 6), 4) hearts in which preconditioning was substituted by administration of 10 microM phenyl-isopropyl-adenosine (PIA) over 5 min, and 5) hearts in which preconditioning was substituted by the administration of 1.5 mg adenosine over 5 min. Hemodynamic results show significant improvement of the postischemic recovery of left ventricular developed pressure (DP) by preconditioning. SPT pretreatment did not reverse the improvement of recovery, obtained by preconditioning, whereas PIA treatment could not mimic preconditioning. Adenosine treatment caused some improvement of recovery of DP, but which remained lower compared to that caused by preconditioning. The contracture developed during ischemia persisted in control hearts, whereas contracture disappeared in non-treated preconditioned hearts. SPT did not prevent the decrease in contracture by preconditioning although values remained slightly higher than in the non-treated preconditioned hearts. PIA did not substitute for preconditioning in preventing contracture. In the adenosine treated group, some decrease of contracture occurred during reperfusion, but values remained significantly higher than in preconditioning. We conclude that receptor A1 activation is not the main mechanism underlying improved functional recovery conferred by preconditioning since an A1 receptor blocker (SPT) cannot reverse the effect of preconditioning and an A1 receptor agonist (PIA) cannot mimic it. Administration of exogenous adenosine reduces functional impairment to a certain extent, but less than preconditioning.